Hydrostatically propelled harvesting machines and the like, where a significant time delay exists between a change in the travel speed and the effect of the change in speed on engine loading or machine function, typically require speed controls. Known speed controls for harvesting machines have been employed to reduce the travel speed of the machine at a fast rate when the engine begins to lug, which may be caused by the machine encountering a sudden increase in crop density or toughness. However, because of the delay between a change in speed and the effect thereof on machine loading, regenerative instability in the system may occur with such speed controls if the travel speed of the machine is allowed to recover at the same fast rate. For example, in the case of a combine harvester, there is a significant delay between the time the header picks up crop material and the time at which the crop material reaches the thresher, which is the primary source of engine load variation. If the travel speed of the combine is allowed to recover or increase at a fast rate after the travel speed has been reduced, because of the crop transport delay between the header and thresher, the travel speed may increase above a desired equilibrium speed causing the header to pick up an excessive amount of crop material before the engine is loaded sufficiently by the thresher to limit the travel speed. When the excessive crop flow reaches the thresher, the engine will lug, resulting in a slowdown of the combine and repetition of the cycle.
In order to prevent such regenerative instability, speed controls for harvesting machines have been known which slow the travel speed of the harvester at a fast rate in response to lugging of the engine and subsequently allow the travel speed to recover at a slow rate to prevent regenerative instability or surging of the speed. The recovery time for such known speed controls may be on the order of 10-20 seconds after the travel speed has been slowed due to engine lugging. The slow recovery rate is not only annoying for the operator of the machine, but also reduces harvesting productivity.